Process for preparing high-whiteness hydrophobic precipitated silica with ultralow moisture absorption

ABSTRACT

A hydrophobic precipitated silica is prepared by a process comprising:
         a) preparing a mixture of an organopolysiloxane derivative and a precipitated silica;   b) conditioning the mixture obtained at a temperature ranging from 10 to 150° C. for a period ranging from 0.5 to 72 h, and   c) conducting oxidative heat treatment under fluid conditions at a temperature more than 300° C. with an oxidizing gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing a hydrophobicprecipitated silica featuring extremely low water absorption, a highlevel of whiteness, and properties of low thickening in silicone rubberformulations with a high reinforcing action in the siliconevulcanizates.

2. Description of the Background

The treatment of finely divided solids, metal oxides, and silicates withorganosilicon compounds, such as organopolysiloxanes, is known, forexample, as disclosed in DE 30 85 905. The process involves a heattreatment process which is conducted under an inert atmosphere ofnitrogen. Additionally, hydrophobic silicates are known, for example, asdescribed in DE 24 35 860, DE 22 42 728, and DE 25 13 608.

In these documents, hydrophilic silicates and precipitated silicas arehydrophobicized by reacting them with organosilicon compounds. Examplesof hydrophobicizers used include organohalosilanes andorganopolysiloxane compounds.

DE 26 28 975 and DE 27 29 244 describe preparing hydrophobic silicas byreacting a hydrophilic precipitated silica featuring low waterabsorbency with silicone oil or dimethyldichlorosilane, respectively. Inthe process described in DE 26 28 975, the reaction is conducted withthe hydrophobicizer (silicone oil) being added to the dry precipitatedsilica. In the process described in DE 27 29 244, the hydrophobicizer(dimethyldichlorosilane) is introduced directly into the precipitatedsilica suspension. In both cases, the hydrophobicizing step is followedby heat treatment at elevated temperatures, specifically ranging from200 to 400° C.

A disadvantage of this process is that the precipitated silica thushydrophobicized becomes discolored at the required process temperatures.The discoloration of this silica is particularly inconvenient when it isadded to silicone formulations; that is, when these hydrophobicprecipitated silicas are added to silicone rubber formulations or todefoamer mixtures based on silicone oil.

As a measure of the discoloration it is possible to employ the valueknown as reflectance. In measuring the reflectance of hydrophobicprecipitated silica, the diffuse reflection power of a sample isinvestigated. The higher the diffuse reflection power of a sample, thehigher its reflectance and thus the higher the whiteness of the sample.

Precipitated silicas generally have a reflectance of not more than 97%.(In this regard, an object of the present invention is to hydrophobicizesilicas in such a way that, ideally, the reflectance of the originalsilica is retained.) Discoloration occurs, in particular, when theprecipitated silicas are strongly hydrophobicized; that is, have a highmethanol wettability and high carbon loadings It is precisely theseproperties, however, which are in many cases (in silicone rubberformulations, for example) desired.

A further disadvantage of the known processes is that only a limitedamount of hydrophobicizer can be attached covalently to the silica.Particularly in silicone rubber formulations, however, high carbonloadings are desired, since they permit decisive improvements in therheological properties, such as the thickening, i.e., low yield pointand low viscosity, of the compounds.

As a measure of the thickening of the treated silica, it is convenientto use what is termed the DBP number. The DBP number indicates theabsorption capacity of a silica for DBP. The measurement technique showsthe amount of dibutyl phthalate, in grams, on a sample of 100 g, atwhich a massive increase in force in the compounder is observed.

It is also not possible to achieve high carbon loadings by usingdiorganodichlorosilanes, or hydrolysis products ofdiorganodichlorosilanes, or with corresponding diorganopolysiloxanes inexcess to the silanol groups present, because the totality of theorganosilicon compounds is no longer attached covalently to the silica.In hydrophobicized silicas for fractions of hydrophobicizing agent thathave not been covalently attached, there is a risk that these moleculesmay have a marked mobility, which in many applications can be verydetrimental (e.g., in silicone rubber applications for medical purposesor for articles that are safe in food contact, such as pacifiers, etc.).

A further disadvantage of the prior art processes is that the relativelylow carbon contents of less than 3.1% lead to hydrophobic silicas whichhave a strong thickening action in silicone rubber formulations. DE 2628 975 lists data on the testing of hydrophobic precipitated silica insilicone rubber formulations, in which the hydrophobic precipitatedsilica is used in increasing weight fractions. From these data it isclear that, at a level of just 15% of hydrophobic silica in the rubber,the self-leveling properties of the silica disappear and that, at 20%,flowable compounds are no longer obtained. All tables clearly indicatethat all of the mechanical properties are improved as the filler contentgoes up. It would therefore be desirable to prepare silicone rubberformulations which include high fractions of hydrophobic silicas, forimproving the mechanical properties, but which at the same time arestill flowable.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a processfor preparing a hydrophobic precipitated silica which has a high,covalently attached carbon loading, low water absorption, alow-thickening effect with good reinforcer properties in silicone rubberformulations, and a high level of whiteness.

Briefly, this object and other objects of the present invention ashereinafter will become more readily apparent can be attained by aprocess of preparing a hydrophobic silica which has the requiredproperties by:

a) preparing a mixture of an organopolysiloxane compound and aprecipitated silica,

b) conditioning the mixture at a temperature ranging from 10 to 150° C.for a period of time ranging from 0.5 to 72 h, and

c) conducting oxidative heat treatment under fluid conditions at atemperature more than 300° C. with an oxidizing gas.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows the methanol wettability of a hydrophobicized silica of thepresent invention; and

FIG. 2 shows the methanol wettability of a conventional hydrophobicizedsilica which has not been heat treated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the invention is suitable for use on the industrialscale; that is, it consistently yields reproducible hydrophobicizationresults in producing a silica of a high whiteness, and is capable ofbeing conducted both continuously and in a batchwise manner. The presentprocess distributes a polysiloxane on a hydrophilic silica withsubsequent conditioning and an oxidative heat treatment in, for example,a fluidized bed reactor.

Conditioning of the treated silica is preferably conducted by means of aheat treatment at a temperature ranging from 100 to 150° C. over aperiod of time ranging from 0.5 to 2 h and/or at a temperature rangingfrom 50 to 80° C. for a period of time ranging from 15 to 30 h. It islikewise possible to store the treated silica at room temperature, i.e.,approximately 20° C. for at least 48 h.

The hydrophobic precipitated silica prepared by the process of theinvention has the following properties:

carbon content >3.1% methanol wettability  >60% reflectance  >94%BET/CTAB ratio >1 and <3 DBP absorption <230 g/100 g BET surface area50–110 m²/g CTAB surface area >30 m²/g water vapor absorption at 30° C.and 1.1 ± 0.2% 30 AH* water vapor absorption at 30° C. and 1.4 ± 0.3% 70AH* *AH = ambient humidity

The ranges of preference specified may be adjusted independently of oneanother.

The hydrophobic silicas of the invention may additionally, eachindependently of one another, be characterized by the followingproperties:

Sears number <1.6 pH 5–9 water content <2% conductivity <150 μS loss onignition >3%

The conductivities of the treated silicas may be below 100, 60, 30, oreven 20 μS.

The process provides a silica which:

-   -   has an extremely high whiteness (reflectance above 94%),    -   exhibits no discoloration in air even at temperatures above 300°        C.,    -   has an extremely low moisture absorption, at the same level as        pyrogenic silicas,    -   has a highly homogeneous hydrophobicization, i.e., a steep        methanol wettability curve,    -   has a high methanol wettability (>60%),    -   has a high level of firmly attached carbon (>3.1%),    -   exhibits a low thickening action in silicone rubber,    -   contains virtually no ionic impurities, i.e., low conductivity        <150 μS, and    -   contains no surfactants, emulsifiers or organic solvents which        might lead to discoloration at elevated temperatures.

The process for preparing the silica of the invention makes it possibleto achieve homogeneous distribution of the hydrophobicizer on the silicawhile avoiding the use of solvents (except for water), emulsifiers,surfactants or other surface-active agents in the hydrophobicizer, sothat the resulting silica combines low-thickening properties with theabsence of discoloration.

The effective distribution of the hydrophobicizer and the high degree ofhydrophobicization of the precipitated silica of the invention result insilicone rubber formulations, for example, of very low thickening, whichis unimpaired even by prolonged storage, in conjunction with goodmechanical and optical properties in the vulcanizates.

Silicas of the invention exhibit a steep methanol wettability; in otherwords, they undergo homogeneous hydrophobicization (FIG. 1). FIG. 2shows the methanol wettability of customary hydrophobic silicasheat-treated without the conditioning of the invention. The silica ofthe invention is preferably prepared with a polysiloxane, so that afterheat treatment its only organic radicals are methyl groups, this goinghand-in-hand with a very high thermal load-bearing capacity (>300° C.).The ingress of air during treatment does not lead to discoloration ofthe product.

The process of the invention makes it possible to effecthydrophobicization, particularly of silicas with a low silanol groupdensity. (The measure which indicates the amount of silanol groups isthe Sears number, i.e., the alkali consumption of an acid-basedtitration. Together with the BET surface area it is then possible tocompare silanol group densities relatively.) The present process makesit possible to achieve a high whiteness (>94% reflectance) in theresulting silica of the invention.

The water absorption of the hydrophobic precipitated silica of theinvention, which is very low for a precipitated silica, is made possibleby the selection of a hydrophilic precipitated silica with a very lowsilanol group density and through very homogeneous hydrophobicizationwith organosilicon compounds. The measure used for the silanol groupdensity is the Sears number in proportion to the BET surface area.

The reaction conditions employed in the process of the invention do notlead to carbonization and thus lead to virtually no discoloration of theproduct. For this reason it is important that the hydrophilic basesilica contains no organic impurities, since otherwise discoloration ofthe product increases. Since the cleavage products which form as aresult of the heat treatment, and which are gaseous under the processconditions, may bring about a certain degree of discoloration even in anoxidizing atmosphere, it is important to remove these cleavage productsfrom the product by means of a sufficient throughput of gas.

The use of liquid polysiloxane, preferably polydimethylsiloxane ofpreferably 30–100 cSt, permits optimum distribution on the base silica.Under the oxidative reaction conditions, the polydimethylsiloxanes usedare able to undergo resinification. This has the great advantage thatthe hydrophobicizer can be distributed on the silica in liquid form andthen fixed. The amount of bound carbon can be greatly increased by theoxidative heat treatment.

High carbon loadings and high methanol wettabilities bring aboutdecisive improvements in the properties of silicas in silicone rubberformulations. Further reduction in moisture absorbency permits thevulcanization of rubber at atmospheric pressure and at temperaturesabove 100° C. in silicone rubber formulations, since no disruptive vaporbubbles appear in the vulcanizate. The high carbon content silicasexhibit substantially improved Theological properties in silicone rubberformulations; that is, they have only a low thickening action, andexhibit low yield points. This low thickening action makes it possibleto prepare flowable silicone rubber formulations which are filled withwell above 20% of hydrophobic precipitated silica and yet can still beprocessed by injection molding. Additionally, the higher filling levelof treated silica leads to markedly improved mechanical properties inthe vulcanizates.

The hydrophobic precipitated silica prepared by the process of theinvention may therefore be used in the following applications:

1. As a Filler in Silicone Rubber Formulations.

The silica of the invention can be used without furtherhydrophobicization in all types of silicone rubber. The low waterabsorption suppresses evolution of vapor in high temperaturecrosslinking systems such as LSR and HTV silicone rubber, and allowspore-free vulcanizates. The low DBP number leads to low compoundviscosities, which can be used to advantage in LSR and RTV2 compounds.The compounds possess a high level of stability on storage; that is, thephenomenon referred to as after stiffening is suppressed. Because of thelow moisture absorption, the silica of the invention can also be used insystems which cure by atmospheric humidity such as RTV1 compounds. Thecompounds likewise have a high level of stability on storage. Since themoisture content of the silica of the invention is greatly reduced,undesired hardening during storage is suppressed.

Because of the high whiteness of the silica, attractive whitevulcanizates can be produced. The low water absorption of the silicaresults in low moisture absorption in the vulcanizates. This produceshigh electrical resistance and an increase in aging stability,especially at high temperatures. These properties are particularlyuseful in electrical insulators and in seals.

2. As a Defoamer Component.

DE 28 29 906, U.S. Pat. No. 4,377,493, DE 34 11 759, U.S. Pat. No.4,344,858, and WO 95/05880 teach that hydrophobicized silicas can beused in defoamer formulations. Advantageous for high-level defoamerperformance here are the high hydrophobicity and a high surface area,readily accessible even to relatively large molecules, of the silicaprepared by the procedure of the invention. The high hydrophobicity ofthe silica of the invention, furthermore, ensures high alkali resistanceand results in much higher service lives particularly in stronglyalkaline media.

The high reflectances of the silicas ensure appealing defoamerformulations free from discoloration, particularly in formulations basedon mineral oil and silicone oil.

3. As a Free-flow Agent.

It is known (for example, as disclosed in Degussa AG brochure series,Fallungskieselsauren und Silikate [Precipitated silicas and silicates],1984) that hydrophobicized silicas can be used as free-flow auxiliaries.Because of its low water absorption, the silica prepared by the processof the present invention is particularly suitable as a free-flowauxiliary for substances that are sensitive to hydrolysis. Here again,the high reflectances of the silicas of the invention are an additionaladvantage.

The silica of the invention may also be used as a carrier substance,particularly for insecticides, as an insecticide per se, as anantiblocking auxiliary, or as a filler in silicone rubber mixtures whichcure by atmospheric humidity.

The hydrophobic precipitated silicas are prepared in three steps:

-   -   First, a liquid polysiloxane derivative is initially        distributed, physically, on the silica surface. When this        initial distribution is conducted in aqueous media, i.e.,        suspensions or silica having a water content of more than 70%,        the silica is typically unstable. It must therefore be isolated        quickly by filtration following the initial distribution, and/or        subjected to short term drying (in a spin-flash drier or nozzle        tower drier, for example). This conserves the distribution of        the organopolysiloxane droplets on the silica and prevents        separation into water, silicone oil, and silica.    -   Subsequently, in a controlled conditioning step—process step        b)—the distribution of the hydrophobicizer is improved further        and engagement of the polysiloxane derivative with the silica        surface is achieved. This state of distribution is stable even        in aqueous media. Following process step b), there is no longer        any separation between the polysiloxane derivative and the        silica. At carbon contents ≧3.1, the conditioned silicas can be        adjusted steplessly to a methanol wettability of up to 55%. The        BET/CTAB ratio after this step is <1. The binding of the        polysiloxane to the silica is thought to be a result of the        formation of multiple hydrogen bonds between the siloxane        bridges of the polysiloxane molecules and the silanol groups on        the silica surface.    -   The conditioning step is followed by a heat treatment in an        oxidizing atmosphere, which suppresses discoloration, ensures        covalent binding of the hydrophobicizing agent, and    -   probably as a result of the formation of gaseous cracked        products—increases further the distribution of the        hydrophobicizer on the silica. Heat treated silicas, with a        lower carbon content than the corresponding conditioned silica,        have a higher methanol wettability. Heat treatment in an        oxidizing atmosphere assists the resinification of the        polysiloxanes, so that much larger amounts of hydrophobicizer        can be anchored covalently on the silica. The BET/CTAB ratio has        turned around and is now >1.

As the organopolysiloxane compound of the invention, it is possible touse any organosilane or organohalosilane which is commonly used tohydrophobicize precipitated silicas.

Step a) of the process of the invention can be conducted with thefollowing variants:

-   -   Addition of organopolysiloxane compound to a precipitated silica        having a water content ranging from 1 to 80% by weight,        preferably from 20 to 60% by weight.    -   Addition of the organopolysiloxane compound to a dispersion of        the precipitated silica, i.e., following precipitation of        silicate with an acid, for example, using a Rhein-Hütte mixer or        Kotthof-Mischsirene or Ultra-Turrax. This necessitates rapid        filtration and/or accelerated drying after the reaction.    -   Addition of the organopolysiloxane compound to a precipitated        silica having a water content ranging from 70 to 99% by weight,        with subsequent isolation of the solid from the water. Isolation        can be effected by filtration, nozzle tower, spin-flash, or        other short term drying. The higher the water content, the more        quickly isolation of the solid should be conducted. Separation        should be avoided.    -   Simultaneous supplying of the precipitated silica or hydrous        silica and the organopolysiloxane derivative to a spin-flash        drier.    -   Mixing of dry precipitated silica with polysiloxane, in a        Gericke mixer, for example.

An alternative possibility is first to prepare a masterbatch, i.e., aconditioned precipitated silica, prepared by process steps a) and b),and then mixing the conditioned silica with a (hydrophilic), hydrousprecipitated silica, e.g., filtercakes, silica suspensions or silicadispersions.

The water content of the hydrophilic precipitated silica may vary withinthe ranges mentioned above.

The base silica may be coated in a weight ratio, for example, rangingfrom 1:1 to 1:3 with silicone oil, e.g, DOW CORNING (R) 200 FLUID 50 CS(50 mPas dimethyl-polysiloxane terminated with trimethylsilyl groups,carbon content of approximately 33%) (step a)). The resulting powder isconditioned at a temperature of more than 100° C. for half an hour, forexample. The conditioning (step b) here. is conducted until theresulting material is wettable by water (methanol wettability <20;regarding the definition of methanol wettability see the measurementtechnique section), but which, when introduced into water no longerexhibits any separation between silica and silicone oil (if step c),follows directly on from step b), a methanol wettability >20 ispreferred). Mixing of this masterbatch, (e.g., 50% by weight silica and50% silicone oil) with aqueous silica dispersions or silica suspensionsproduces stable mixtures in which the silicone oil no longer separatesfrom the silica. The total mixture typically contains one part by weightof silicone oil, about 4–8 parts by weight of silica, and 20–60 parts byweight of water. In order to prepare such a suspension, for example, themasterbatch (e.g., 50% silica and 50% silicone oil) can be mixedthoroughly with about 10–16 times the amount of filtercake (solidscontent approximately 20%) and about 10–20 times the amount ofadditional water. The advantage of this procedure is that thewater-wettable masterbatch, which contains up to 75% of hydrophobicorganopolysiloxane, can be dispersed directly in silica precipitationsuspensions or silica feeds, very finely and stably, without the need touse emulsifiers or surfactants. After such a mixture has been dried, orfiltered and then dried, the organopolysiloxane-containing silica thusobtained can be conditioned again (step b).

These steps can be carried out individually, where appropriate withmilling beforehand. Milling should not, however, be conducted beforecoating a). It is also possible to carry out two or more of thesevariants, that is, identical or different variants in succession. Thefollowing embodiments of the process of the invention are conceivable:

-   -   One of steps a), b), and c) is performed a number of times (from        2 to 5 times) in succession.    -   Steps a) and b) are conducted a number of times (from 2 to 5        times) in succession.    -   All steps a), b), and c) are conducted a number of times (from 2        to 5 times) in succession; in other words, the process is run        through a number of cycles.

Process step b) is preferably conducted by heat treatment at 100–150° C.over the course of from 0.5 to 2 hours. After conditioning, the partlyhydrophobicized silica present may have a methanol wettability of 20% ormore. Fundamentally, a distinction may be made between wet and dryhydrophobicization.

The definition of wet hydrophobicization is that the silicate startingmaterials are aqueous silica suspensions, silica feeds, or high watercontent silica filtercakes, which are coated with the correspondinghydrophobicizers, as described, for example, in DE 27 29 244 forprecipitation suspensions with organohalosilanes.

The definition of dry hydrophobicization is that the silicate startingmaterials are silica powders with different moisture contents of from 1to 75%, which are coated with the corresponding hydrophobicizers. Thisprocess is described, for example, in DE 26 28 975.

The silica of the invention is prepared using organopolysiloxanecompounds. It is, however, also possible to use other silicone compoundswhich react to give organopolysiloxanes under the selected reactionconditions, for example, dichlorodimethylsilane in an aqueousenvironment.

Suitable hydrophobicizing agents employed in the invention includeorgano-polysiloxane compounds or their precursors, for example, thosewith the composition R_(4−n)SiX_(n), wherein n=1, 2 or 3;[SiR_(x)X_(y)O]_(z), wherein 0≦x≦2, 0≦y≦2, 3≦z≦10 and x+y=2;[SiR_(x)X_(y)N]_(z), wherein 0≦x≦2, 0≦y≦2, 3≦z≦10 and x+y=2;SiR_(n)X_(m)OSiR_(o)X_(p), wherein 0≦n≦3, 0≦m≦3, 0≦o≦3, 0≦p≦3, n+m=3 ando+p=3; SiR_(n)X_(m)NSiR_(o)X_(p), wherein 0≦n≦3, 0≦o≦3, 0≦m≦3, 0≦p≦3 andn+m=3, o+p=3; SiR_(n)X_(m)[SiR_(x)X_(y)O]_(z)SiR_(o)X_(p), wherein0≦n≦3, 0≦m≦3, 0≦x≦2, 0≦y≦2, 0≦o≦3, 0≦p≦3, 1≦z≦10,000, n+m=3, x+y=2 ando+p=3). These compounds may be linear, cyclic, and branched silane,silazane and siloxane compounds. R may comprise alkyl and/or arylradicals, which may be substituted by functional groups such as thehydroxyl group, the amino group, polyethers such as ethylene oxideand/or propylene oxide, and halide groups such as fluoride. R may alsocontain groups such as hydroxyl, amino, halide, alkoxy, alkenyl,alkynyl, and allyl groups, and groups containing sulfur. X may comprisereactive groups such as silanol, amino, mercapto, halide, alkoxy,alkenyl, and hydride groups.

Preference is given to linear polysiloxanes having the compositionSiR_(n)X_(m)[SiR_(x)X_(y)O]_(z)SiR_(o)X_(p), wherein 0≦n≦3, 0≦m≦3,0≦x≦2, 0≦y≦2, 0≦o≦3, 0≦p≦3, 1≦z≦10,000, n+m=3, x+y=2 and o+p=3, in whichR is preferably represented by methyl.

Particular preference is given to polysiloxanes having the compositionSiR_(n)X_(m)[SiR_(x)X_(y)O]_(z)SiR_(o)X_(p), wherein 0≦n≦3, 0≦m≦1,0≦x≦2, 0≦y≦0.2, 0≦o≦3, 0≦p≦3, 1≦z≦1,000, n+m=3, x+y=2 and o+p=3, inwhich R is preferably represented by methyl. It is also possible,however, to use polysiloxanes of low volatility which contain nofunctional groups in the process of the invention.

Because of the presence of certain functional groups in polysiloxanecompounds, salts or low molecular weight substances such as NH₃, amines,alcohols, and the like may be formed, which can lead to disruptiveimpurities. An important exception here is constituted bysilanol-functionalized polysiloxanes, since the only impurity formedhere is water, which is easy to remove under the selected processconditions.

With preference, the hydrophobicizer may comprise a methyl-terminatedpolydimethylsiloxane, in particular one having a viscosity of 30–100 inmPas, preferably 40–60 mPas. An example of a suitable polysiloxane oilis DOW CORNING (R) 200 FLUID 50 CS.

Since the aforementioned hydrophobicizers are compounds of lowvolatility, an important part in the initial distribution of thehydrophobicizers on the silica surface is played by capillary forces anddiffusion events at the liquid/solid phase boundary.

Even if the hydrophobicizers of preference exhibit a certain volatilityin the course of thermal treatment, the liquid/solid distribution isstill important. For this reason, a distinction is made here betweenphysical, initial distribution, conditioning, and heat treatment.

The heat treatment, i.e., process step c), is conducted at a temperatureof at least 300° C., preferably above 350° C., with very particularpreference above 360–370° C., with an oxidizing gas. Suitable gases foruse in the process include air, Cl₂, NO_(x) (NO₂, N₂O₅, NO, N₂O), O₃,O₂, Br₂, F₂, or mixture of these gases with another inert gas such asCO₂, N₂ or burner waste gases, in each case preferably at not less than1% by volume.

Additionally, the oxidizing gas may optionally contain up to 80%,preferably up to 50%, with particular preference 20–40%, by volume ofwater.

In every case, a good gas throughput must be ensured; as far aspossible, the gas must reach every silica particle. This is achieved bythe fluidic conditions in the process of the invention.

Fluidic conditions such as, for example, fluidized layers or a fluid bedin accordance with the definition in Römpp Lexikon Chemie, 2nd edition,1999, can be set, for example, in a suspended bed, moving bed, fluidizedbed and/or turbulent bed.

It is possible to combine two or more of these types of reaction. Theprocess may be operated batchwise or continuously.

By way of example, the text below gives further details of theconditions in a fluidized bed reactor.

Heat Treatment in a Fluidized Bed:

Heat treatment of the polysiloxane-treated silica under oxidizing, i.e.,bleaching reaction conditions, can be conducted in a fluidized bed inthe presence of oxygen. Since precipitated silica powders, includingpolysiloxane-treated silicas, are typically readily free-flowing andfluidizable, they can easily be heat-treated in a fluidized bed. Thedecisive advantages of the fluidized bed reactor lie in the homogeneousdistribution of material and of temperature in the fluidized bedreactor. Even in the case of predominant wall heating and temperaturesabove 300° C., wall temperature and product temperature differ by only afew degrees (typically only 2–4° C.) because an active fluidized bed,i.e., a fluidized bed operated at above its fluidization point, exhibitsoptimum thermal conductivity.

With conventional wall-heated reactor types—or with types of reactors inwhich the heating rods contact the product directly—this homogeneoustemperature distribution is difficult to realize in the case of athermally insulating product such as silica. Particularly in the case ofreactor types with heating rods, or when it is attempted to heat thesilica from room temperature to more than 300° C. in less than 0.5 h,instances of severe local overheating may occur. In such cases theoxidizing gas, oxygen for example, no longer has a bleaching effect butinstead has a carbonizing and thus a discoloring effect. Conventionalreactors normally house a stirrer mechanism in order to achieve morehomogeneous distribution of materials and temperature. Because silicapossesses abrasive properties, metal abrasion can occur in this case andleads to discoloration of the silica.

In fluidized bed reactors, silicas can be heat treated withoutdisruptive metal abrasion being in evidence.

The homogeneous distribution of materials and distribution oftemperature make it possible to conduct heat treatment at relatively lowprocess gas throughputs (0.5–5 m³/[h·kg]. This leads to relatively highconcentrations of elimination products and hence to effectivehydrophobicization by the elimination products) and, for example, oxygencontents of 4–7% by volume. In this case the concentration range inwhich the oxygen exhibits its bleaching action extends close to thecritical oxygen concentration of the explosion limit of theoxygen/silicone oil elimination products mixture. If distribution is nothomogeneous, local excess oxygen may lead locally to an ignitablemixture and thus to explosion. With the fluidized bed, the desiredprocess conditions can be maintained safely. Furthermore, in thefluidized bed it is possible to realize even very high process gasthroughputs (up to 50 m³/[h·kg]). These high gas throughputs allow asafe heat treatment even at oxygen concentrations of more than 10%oxygen content.

The heat treatments are preferably conducted with relatively low processgas throughputs (0.5–5 m³/[h·kg]) at oxygen contents of 4–7% by volume.This leads to relatively high elimination product concentrations (thusguaranteeing effective hydrophobicization via the gas phase), whilenevertheless allowing excess elimination products to be discharged. Atthese oxygen concentrations, the elimination products are almost—but notcompletely—free of discoloration and must therefore be expelled ascompletely as possible.

Heat treatment of the silica powder in the fluidized bed reactor can beconducted batchwise or continuously. In continuous operation, thereactor should be operated in such a way that the fraction of silicaparticles exposed to the operating temperature for less than 15 minutesis as small as possible. This can be done, for example, by means of anappropriate arrangement of weirs. The preferred heat treatment times are0.25–4 h at temperatures of 330–400° C.

The reactor size may range from laboratory scale (100 g range/charge)via pilot plant size (100 kg range/charge) through production scale(>100 kg/charge).

This type of reactor has the advantage that it can be operated withaverage particle sizes of 5–500 μm. This means that it is not absolutelynecessary here to employ a grinding operation following the heattreatment.

Following the conditioning step and/or heat treatment, thehydrophobicized silica can be in all cases ground. Milling before thecoating step a), however, is not appropriate, and leads to low-gradeproducts with inhomogeneous hydrophobicization.

Optional milling gives a silica having a d_(4.3) of 8–25 μm, preferably8–15 μm.

The use of the precipitated silicas prepared in accordance with theinvention as a filler in silicone rubber formulations, as defoamerauxiliaries or as free-flow auxiliaries is likewise provided by thisinvention.

In order to fully develop the mechanical properties of silicone rubberformulations, the formulations require active reinforcing fillers. It iscommon to use highly dispersed silicas. Because of the ease ofmechanical processing of LSR (liquid silicone rubber) formulations,especially in injection molding processes, HTV (high temperaturevulcanizing) silicone rubber formulations are increasingly beingreplaced by LSR mixtures. The reinforcing filler must bring about goodmechanical properties in the vulcanizate without impairing therheological properties of the silicone rubber formulations. Aftercompounding, the silicone rubber formulations must be flowable andshould not undergo after stiffening even following prolonged storagetimes.

HTV and LSR formulations are processed at temperatures well above 100°C. At such temperatures, hydrous fillers may lead to disruptiveformation of vapor bubbles in the silicone formulation. In the case ofsilicone rubber formulations which cure by atmospheric humidity, anexcessively high water content in the filler results in unwanted curingin the course of storage. Accordingly, the water absorptioncharacteristics, i.e., the amounts of water adsorbed at differentrelative atmospheric humidities, constitutes a measure of theprocessability of the filler.

The problem of vapor bubble formation occurs particularly with thehydrophilic precipitated silicas. Even hydrophobic precipitated silicasdo not, typically, exhibit the low water absorption characteristics ofthe pyrogenic silicas.

The hydrophobic precipitated silica prepared by the process of theinvention, however, exhibits water absorption characteristics comparablewith those of fumed silicas, is unaffected by discoloration, and alsohas low-thickening properties in silicone rubber formulations.

These properties are derived from the nature of the base silica used andfrom the nature of the hydrophobicization. The base silica is preferablya precipitated silica which has a very low silanol group density (Themeasure of the silanol group density is the Sears number taken togetherwith the BET surface area). The low silanol group density of the basesilica is also manifested in a low loss on ignition of 3.0±0.5 at a BETsurface area of about 160 m²/g.

For silicone rubber mixtures which are processed at temperatures ofalmost 200° C. with ingress of air, it is important that there are noorganic constituents on the silica which might undergo discolorationunder the influence of oxygen at these temperatures. Organosiliconcompounds containing exclusively methyl, phenyl, fluorocarbon orhydrofluorocarbons as organic radicals are extremely temperature-stableeven in the presence of atmospheric oxygen. In order, however, toachieve effective cleavage of the stable siloxane bridges of siloxanecompounds and to bond them covalently to the silica, temperatures above300° C. are required. At these high temperatures, siloxane compounds,especially in the case of precipitated silicas with a low silanol groupdensity, normally lead to discoloration phenomena on the silica. Theprocess of the invention makes it possible to suppress thisdiscoloration These discoloration phenomena are measured by reflectancemeasurements with an optical measurement technique based on diffusereflection. Where the reflectances of silica are >94%, the silica-filledsilicone rubber compound appears pure white. Since the refractiveindices of silica and silicone rubber are close to one another, evenvery small impurities and discolorations in the silica filler becomeclearly visible in the silicone rubber. A reflectance of 93% alreadyleads to a marked discoloration in the silicone rubber, visible with thenaked eye, despite the fact that the silica powder before incorporationappears pure white to the viewer.

By mixing the silica with diorganopolysiloxanes and, where appropriate,further substances at room temperature or only slightly elevatedtemperature, it is possible to prepare compositions which can be curedto give elastomers, following the addition of crosslinking agents whereappropriate. The silica of the invention has a low thickening effect, sothat the compounds can be processed in LSR systems on injection moldingmachines. Mixing can be conducted conventionally, in mechanical mixers,for example.

The silicas are employed preferably in amounts ranging from 5 to 50% byweight, more preferably from 10 to 40% by weight, based on the overallweight of the compositions which can be cured to give elastomers. In thecase of HTV organopolysiloxane elastomers it is possible to use up to50% by weight.

Besides diorganopolysiloxanes, the hydrophobicized precipitated silica,crosslinking agents and crosslinking catalysts, the compositions whichcan be cured to elastomers may of course, where appropriate, includefillers which are conventionally, often or usually, used in compositionsthat can be cured to elastomers. Examples of such substances are fillershaving a surface area of less than 50 m²/g, such as quartz flour,diatomaceous earth, and also zirconium silicate and calcium carbonate,and also untreated fumed silica, organic resins, such as polyvinylchloride powders, organopolysiloxane resins, fibrous fillers, such asasbestos, glass fibers and organic pigments, soluble dyes, fragrances,corrosion inhibitors, agents which stabilize the compositions againstthe influence of water, such as acetic anhydride, agents which retardcuring, such as benzotriazol, and plasticizers, and alsotrimethylsiloxy-endblocked dimethylpolysiloxanes.

The cited combination of physicochemical characteristics of thehydrophobic precipitated silica prepared by the process of the inventionresults in an outstanding reinforcing filler. The equilibrium moisturecontent, much lower than that of the precipitated silicas prepared byknown processes, brings advantages in processing, in the context, forexample, of vulcanization at atmospheric pressure, which producespore-free vulcanizates in comparison with the use of the known, hydratedprecipitated silicas. The optimized pH and the low DBP number lead toperceptibly reduced roller-softening times. The low electrolyte contentin combination with the low moisture content leads ultimately to goodelectrical properties in the vulcanizates. In silicone rubber sealantsthat cure by atmospheric humidity, the low water content of thehydrophobic precipitated silica of the invention gives advantages forthe storage properties of the uncured sealants.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLES

As the silicate starting material, it is preferred to use precipitatedsilicas which possess a very low silanol group density, i.e., a lowalkali consumption/BET surface area ratio, a relatively high CTABsurface area for approximately the same BET surface area, and a highlevel of whiteness and purity.

General Description of a Preparation of a Suitable Silica

Aqueous sodium silicate solution (waterglass), and sulfuric acid isadded with stirring into a reaction vessel pre-charged with water.During the addition, an alkaline pH is maintained. The silicaprecipitates from the reaction mixture, which is then acidified to pH2–5 and filtered. The solid product is washed with water and dried.

Any silica with the following properties is suitable as startingmaterial for the hydrophobic process according to the invention.

BET surface area (m²/g) 50–170 CTAB surface area (m²/g) 50–170 Loss onignition based on the substance Dried 2 h/105° C. (DIN 55921) (%) ≦3.5%pH 5% (methanol/aqueous solution) (DIN 53200)  5–9 Conductivity (in 5%aqueous dispersion) (μm) <500 μS Tapped density >200 g/l Sears Number<13

The base silica and the polysiloxane are mixed until a defined carboncontent is obtained; in other words, the mixing ratio is a function ofthe arithmetic proportion for setting the required carbon content.

1. Measurement Techniques

1.1 Methanol Wettability

Silicas whose surfaces have been modified with nonhydrolyzable organicgroups are usually not wetted by water.

These hydrophobic silicas can, however, be wetted by a methanol/watermixture. The fraction of methanol in this mixture—expressed as apercentage by weight—is a measure of the hydrophobicity of modifiedsilica. The higher the methanol fraction, the better thehydrophobicization of the substance.

Procedure

A 200 mg amount of each hydrophobic silica or silicate sample is weighedinto 6 centrifuge tubes each with a capacity of 15 ml, and each of thetubes is filled with 8 ml of a methanol/water mixture of ascendingmethanol concentration. The methanol concentration of the mixtures isguided by the anticipated methanol wettability. The centrifuge tubes aretightly sealed and then shaken vigorously (10 up-and-down motions). Toseparate the wetted silica/silicate fractions, the tubes are thencentrifuged at 2,500 rpm for 5 minutes. The wetted fractions form asediment whose volume can be read on the scale on the centrifuge tubes.On a graph, the sediment volumes are plotted against the methanol/watermixture concentration. These individual points produce a curve whoseposition and steepness characterizes the degree of hydrophobicization ofthe sample under analysis.

-   Apparatus: Precision balance-   Centrifuge-   Centrifuge tubes, graduated-   Dispensettes    1.2 DBP Absorption

The DBP absorption (DBP number), which is a measure of the absorbency ofthe precipitated silica, is determined as follows:

The dibutyl phthalate number is determined using the Brabenderplastograph. The DBP number is a measure of the absorbency of apulverulent product for liquid. The absorbency is dependent on themoisture content, the particle size, and the amount of materialanalyzed.

Apparatus and Reagents

-   Brabender plastograph with plotter-   Multi-Dosimat E 415 (50 1) from Metrohm-   Dibutyl phthalate    Procedure

A 12.5 g amount of silica is introduced into the kneader of theBrabender plastograph. With continued mixing (kneader paddle speed 125rpm), dibutyl phthalate runs into the mixture at a rate of 4 ml/minute.The force required for incorporation is low. Toward the end of thedetermination, the mixture becomes poorly free-flowing. This fact isdocumented by an increase in the force required to knead the silica/ DBPmixture, which is indicated on the scale. When the scale has moved by300, DBP metering is automatically shut off.

Evaluation

The density of DBP is 1.047 g/ml. The DBP absorption is based on theanhydrous, dried substance. When using precipitated silicas ofrelatively high moisture content, the value must be corrected using thefollowing table if these silicas are not dried prior to thedetermination of the DBP number.

Correction table for dibutyl phthalate absorption—anhydrous

% water % water % water .0 .2 .4 .6 .8 0 0 2 4 5 7 1 9 10 12 13 15 2 1618 19 20 22 3 23 24 26 27 28 4 28 29 29 30 31 5 31 32 32 33 33 6 34 3435 35 36 7 36 37 38 38 39 8 39 40 40 41 41 9 42 43 43 44 44 10 45 45 4646 47

The correction figure corresponding to the water content is added to theexperimentally determined DBP value; for example, a water content of5.8% would mean an add-on of 33 g/100 g for the DBP absorption.

1.3 Particle Size

The particle size is determined using a Malvern Mastersizer in ethanolfollowing ultrasound treatment for 5 minutes. The measurement is madeautomatically and provides the average particle size d_(4.3) from avolume distribution.

1.4 Determination of the Tristimulus Value R_(y) in Accordance with DIN5033

Application

Using the Datacolor 3890 spectrophotometer, the tristimulus value R_(y)is determined for silicas, silicates, and zeolites (powder suspensions).

Analytical Procedure:

The silica to be analyzed is first ground to an average particlediameter of about 8 to 15 μm and then pressed to a tablet using a powderpress. The amount required depends on the fineness of the powder. Theamount of powder introduced is such that the thread of the press closurereaches its last turn.

The samples are placed under the meter, and whiteness measurements R_(y)and R₄₆₀ are selected from the menu of the control computer. After thesample designation has been entered, the space key is operated in orderto start the measurement.

Following entry of the memory code, the measurements are printed.

The values are calculated automatically in accordance with the followingformula:

$Y = {\sum\limits_{400}^{700}{S*(\lambda)*{Y(\lambda)}*{R(\lambda)}}}$where

-   Y(λ) is the standard distribution coefficient,-   S(λ) is the relative spectral radiation distribution of the    illumination source, and-   R(λ) is the spectral reflectance of the sample.    1.5 Determination of the Sears Number of Silicas, Silicates and    Hydrophobic Silicas    1. Application    -   Free OH groups are detectable by titration with 0.1 N KOH in the        range from pH 6 to pH 9.        2. Apparatus    -   2.1 Precision balance accurate to 0.01 g    -   2.2 Memotitrator DL 70, Mettler, equipped with 10 ml and 20 ml        Bürette, 1 pH electrode and 1 pump (e.g., NOUVAG pump, type SP        40/6)    -   2.3 Printer    -   2.4 Titration vessel 250 ml, Mettler    -   2.5 Ultra-Turrax 8000–24000 rpm    -   2.6 Thermostated waterbath    -   2.7 2 dispensers 10–100 ml for metering methanol and deionized        water    -   2.8 1 dispenser 10–50 ml for metering deionized water    -   2.9 1 measuring cylinder 100 ml    -   2.10 IKA universal mill M 20        3 Reagents    -   3.1 Methanol p.A.    -   3.2 Sodium chloride solution (250 g NaCl p.A. in 1,000 ml        deionized water)    -   3.3 0.1 N hydrochloric acid    -   3.4 0.1 N potassium hydroxide solution    -   3.5 Deionized water    -   3.6 Buffer solutions pH 7 and pH 9        4 . Procedure    -   4.1 Sample preparation        -   Mill about 10 g of sample for 60 seconds in the IKA            universal mill M 20.        -   Important: Since only very finely ground samples give            reproducible results, these conditions must be strictly            observed.        -   4.2 Analytical procedure    -   4.2.1 Weigh out 2.50 g of the sample prepared in accordance with        section 4.1 into a 250 ml titration vessel.    -   4.2.2 Add 60 ml of methanol p.A.    -   4.2.3 After complete wetting of the sample, add 40 ml of        deionized water    -   4.2.4 Disperse for 30 seconds using the Ultra-Turrax at a speed        of about 18,000 rpm    -   4.2.5 Rinse particles of sample adhering to the vessel edge and        stirrer into the suspension using 100 ml of deionized water    -   4.2.6 Condition sample to 25° C. in a thermostated waterbath        (for at least 20 minutes)    -   4.2.7 Calibrate pH electrode with the buffer solutions pH 7 and        pH 9    -   4.2.8 The sample is titrated in the Memotitrator DL 70 in        accordance with method S 911. If the course of titration is        indistinct, a duplicate determination is conducted subsequently.        The printed results obtained as follows:-   pH-   V₁ in ml/5 g-   V₂ in ml/5 g    5. Calculation

$V_{1} = {{ml}\mspace{14mu}{KOH}\mspace{14mu}{or}\mspace{14mu}{ml}\mspace{14mu}\begin{matrix}{V_{1} = \frac{V*5}{E}} \\{V_{2} = \frac{V*5}{E}}\end{matrix}}$

-   HCl to pH6/5 g of substance-   V₂=ml KOH consumed to pH9/5 g of substance-   E=initial mass    Principle

First of all the initial pH of the suspension is measured, thenaccording to the result the pH is adjusted to 6 using KOH or HCl. Then20 ml of NaCl solution are metered in. The titration is then continuedto a pH of 9 using 0.1 N KOH.

Sears numbersSi—OH+NaCl→Si—ONa+HClHCl+KOH→KCl+H₂O1.6 Determination of the Tamped Density in Accordance with DIN/ISO787/11Procedure

A 10 g amount of the sample under analysis is weighed accurately to 0.01g on the precision balance, and is introduced into the graduated 250 mlglass cylinder of the jolting volumeter. After 1250 jolts, the volume ofthe tapped material is read.

Calculation:

-   Tapped density:

${g/l} = \frac{E \cdot 1000}{I}$

-   E=initial wright in g-   I=volume in ml    Apparatus:

Precision balance Engelsmann, Ludwigshafen Jolting volumeter 250 mlglass cylinder, Engelsmann, Ludwigshafen graduatedRemarks:

In special cases, the material may be passed through a 500 μm sievebefore weighing, or the initial mass may be increased. This must bespecified in a test report.

1.7 Determination of CTAB Surface Area

1. Application

The method is based on the adsorption of CTAB(N-cetyl-N,N,N-trimethylammonium bromide) on the “external” surface,which is also referred to as the “rubber-active surface”.

The adsorption of CTAB takes place in aqueous solution at pH=9 withstirring and ultrasound treatment. Excess, unadsorbed CTAB is determinedby back-titration with SDSS (dioctylsodium sulfosuccinate solution)using a titroprocessor, the endpoint being given by the maximum cloudingof the solution and determined using a photodiode.

For the calculation, an occupancy of 0.35 nm² per CTAB molecule isassumed.

The determination is made in accordance with ASTM 3765.

With each measurement series, a standard sample of type VN 3 silicashould be tested as well.

2. Reaction Equation: (back-titration)R₁—SO₃ ⁻+⁺N(CH₃)₃R₂→R₁SO₃N(CH₃)₃R₂

NDSS CTAB

3. Apparatus

-   -   3.1 Mill, e.g. IKA, type: M 20    -   3.2 Analytical balance    -   3.3 Magnetic stirrer    -   3.4 Magnetic stirrer rod    -   3.5 Titroprocessor, e.g., METTLER, type DL 55 or DL 70, equipped        with: pH electrode, e.g., Mettler, type DG 111 phototrode, e.g.        Mettler, type DP 550, and burette, 20 ml volume, for SDSS        solution, burette, 10 ml volume, for 0.1 N KOH    -   3.6 titration beakers, 100 ml, made of polypropylene    -   3.7 glass titration vessel, 150 ml volume, closable with snap-on        lid    -   3.8 conical flasks, 100 ml volume, closable with screw lid or NS        stopper    -   3.9 ultrasound bath    -   3.10 pressure filtration device    -   3.11 membrane filter of cellulose nitrate, pore sizes of 0.1 μm,        47 mm ø, e.g., Sartorius type 113 58    -   3.12 pipettes, 5 ml, 100 ml        4. Reagents    -   4.1 Potassium hydroxide solution, 0.1 N    -   4.2 CTAB solution, 0.0151 mol/l 5.50 g of CTAB are dissolved        with stirring (magnetic stirrer) in about 800 ml of warm (about        30–40° C.) demineralized water in a glass beaker, transferred to        a 1 liter graduated flask, made up to the mark with        demineralized water after cooling to 23–25° C., and transferred        to a stock bottle.

Note:

The solution must be stored and the measurement conducted at ≧23° C.,since CTAB crystallizes out below this temperature. The solution shouldbe prepared 10–14 days prior to use.

-   -   4.3 SDSS solution 0.00426 mol/l 1.895 g of SDSS (dioctylsodium        sulfosuccinate) in a glass beaker are admixed with about 800 ml        of demineralized water and the mixture is stirred with a        magnetic stirrer until all of the material has dissolved. The        solution is then transferred to a 1 liter graduated flask, made        up to the mark with demineralized water, and transferred to a        stock bottle.        -   SDSS solution readily undergoes biodegradation. The solution            prepared should therefore be sealed well and should not be            stored for more than 3 months. The concentration of the CTAB            solution is assumed to be exact: 0.0151 mol/l. The            concentration of the SDSS solution should be determined            daily by means of a “blank” titration.            5. Procedure    -   5.1 Blank titration (to determine the concentration of the SDSS        solution)    -   5.2 The consumption of SDSS solution for 5 ml of CTAB solution        should be checked (blank value) 1× per day before each series of        measurements    -   5.1.2 Pipette precisely 5 ml of CTAB solution into titration        beakers    -   5.1.3 Add about 50 ml of demineralized water    -   5.1.4 Titrate with the titroprocessor until the end of titration        Each blank titration should be performed as a duplicate        determination; in the case where values do not agree, further        titration should be conducted until the results are        reproducible.    -   5.2 Adsorption    -   5.2.1 The granulated and coarse samples are ground in a mill        (the beater blade of the mill must be covered)    -   5.2.2 Weight out exactly 500 mg of the ground sample on the        analytical balance to a precision of 0.1 mg    -   5.2.3 Transfer the sample amount weighed out quantitatively to a        150 ml titration vessel with magnetic stirrer rod    -   5.2.4 Add exactly 100 ml of CTAB solution, seal titration vessel        with lid, and stir on a magnetic stirrer for 15 minutes    -   5.2.5 Screw the titration vessel onto the titroprocessor and        adjust the pH of the suspension to 9.0±0.05 using KOH, 0.1 mol/l    -   5.2.6 4-minute treatment of the suspension in the ultrasound        bath    -   5.2.7 Filtration through a pressure filter fitted with a        membrane filter. During adsorption, it must be ensured that the        temperature is held within the range from 23° C. to 25° C.    -   5.3 Titration    -   5.3.1 Pipette 5 ml of filtrate (see section 5.2.7) into 100 ml        titration beakers and make up to about 50 ml with demineralized        water    -   5.3.2 Screw titration beakers onto the titrator    -   5.3.3 Carry out titration with SDSS solution in accordance with        the defined measurement method, until clouding reaches a        maximum. Each titration should be performed as a duplicate        determination; in the case where values do not agree, further        titration should be conducted until the results are        reproducible.        6. Calculation

$V_{1}\mspace{31mu}\begin{matrix}{{m^{2}/g} = {\left( {V_{1} - V_{2}} \right)*\frac{100*E*2*578.435}{V_{1}*1000}}} \\{{m^{2}/g} = {\left( {V_{1} + V_{2}} \right)*\frac{115.687*E}{V_{1}}}} \\{{m^{2}/g} = {\left( {V_{1} + V_{2}} \right)*\frac{115.687}{V_{1}}*5.5}}\end{matrix}$

-   -   V₁=blank sample (ml of SDSS when using 5 ml of CTAB)    -   V₂=consumption (ml of SDSS when using 5 ml of filtrate)    -   E=initial weight g CTAB/l (5.5 g)    -   578.435=occupancy of 1 g of CTAB in m².

The measurement is normally to be given corrected to the anhydroussubstance:

${m^{2}/g} = \frac{{CTAB}\mspace{20mu}{m^{2}/g}*100}{100 - {\%\mspace{14mu} H_{2}O}}$

Where the measured value for the standard sample differs by more than ±3m²/g from the theoretical value, the entire measurement series must berepeated.

7. Notes

-   -   re 1. In the literature, SDSS (dioctylsodium sulfosuccinate) is        also called Aerosol OT. On samples with a pH>9, such as        Extrusil, the pH is measured but not corrected, since the acid        may alter the surface. Prior to beginning the titration, the        phototrode is set to 1,000 mV, corresponding to a transparency        of 100%.    -   re 3. For measuring the different prescribed volumes of the CTAB        solution, it is also possible to use dispensers or piston-stroke        pipettes, provided they are regularly calibrated.    -   re 4. The solutions indicated in sections 4.1 and 4.3 can also        be purchased as ready-to-use solutions. The present supplier is        Kraft, Duisburg.        -   Telephone: 0203-58-3025.        -   Order No. 6056.4 CTAb solution 0.0151 ml/l        -   Order No. 6057.4 SDSS solution 0.00423 mol/l (in 2.5-liter            glass bottles)    -   re 5.2.4 Hydrophobic samples which are not wetted after stirring        are to be dispersed carefully using an ULTRA-TURRAX before the        pH is adjusted, in order to wet them.    -   re 5.2.5 For adjusting the pH it is advisable to use a titrator.        The titration is conducted in accordance with the endpoint        method.    -   re 5.2.7 For filtration, nitrogen from a gas bottle is used; an        entry pressure of 4–6 bar is to be set.    -   re 6. Should it be necessary to repeat a measurement series, it        should be noted in particular that the pH meter used to set the        pH must also be recalibrated.        1.8 Determination of Water Vapor Absorption (Water Vapor        Isotherms)

To determine the water vapor absorption, the sample is exposed todifferent relative humidities at constant temperature (30° C.). Theestablishment of a constant weight is awaited.

To start with, completely dry air, i.e., air with a humidity ofapproximately zero, is used. After the equilibrium weight has beenreached, this weight is chosen as the reference point; in other words,the water vapor absorption at a higher air humidity is expressed as thedifference between the sample weight in completely dry air (followingestablishment of equilibrium) and the sample weight in humid air(following establishment of equilibrium). The humidity of the air isvaried in steps of 10%.

In order to rule out hysteresis effects, both the water adsorption andthe water vapor desorption are measured.

Preparation of Base Silica

A 50.0 m³ amount of water is charged to a reaction vessel. Slowly, 9.2m³ of water glass solution and 0.9 m³ of H₂SO₄ are added with stirringto the initial charge, an alkaline pH being maintained in the mixtureduring the addition. After the end of the addition of water glass andH₂SO₄, the pH of the resulting suspension is within the alkaline range.The suspension is acidified and filtered, and the solid product iswashed with deionized water. The hydrophilic base silica can be dried,preferably by an accelerated drying method. The following data wereobtained of the dried precipitated silica thus prepared.

BET surface area [m²/g] 150–170 CTAB surface area [m²/g] 150–170 Loss onignition based on the substance 3 ± 0.5 Dried 2 h/105° C. (DIN 55921)[%] pH 5% (methanol/aqueous solution)  6–7 (DIN 53200) Conductivity (in5% aqueous dispersion) <150 [μS] Tapped density [g/l] >250

Example 1

The drying and coating of the base silica with silicone oil (e.g., DOWCORNING (R) 200 FLUID 50 CS, carbon content about 33%, methyl-terminatedsilicone oil having a viscosity of 50 mPas) is conducted using aspin-flash dryer. The silica is then conditioned by aging at roomtemperature for at least 48 hours until it has a methanol wettability ofat least 20%. The analytical data for the conditioned silica are givenin Table 1.1.

TABLE 1.1 Analytical data of the conditioned silica Water % 5.2 pH 6.1Conductivity μS 41 N₂ surface area m²/g 84 CTAB surface area m²/g 132Tapped density g/L 317 Alpine SR > 180 μm % 63 Reflectance % 95.9 Ccontent % 4.12 Methanol wettability % >20

The conditioned silica is heat treated in the fluidized bed at differentoxygen contents. The experimental parameters are given in Table 1.2.

TABLE 1.2 Heat treatment in a fluidized bed at different oxygencontents: Exp. 1 Exp. 2 Exp. 3 Heat treatment time [min] 60 60 60Product temperature [° C.] 320–380 320–380 320–380 Oxygen content [%]   0–0.001 4.0–6.0 20–22 C content before heat treatment 4.12 4.12 4.12

The experiments show the correlation between oxygen content anddiscoloration. Only in the case of experiments 2 and 3 is areflectance >94% achieved. In contrast to a silica from experiment 1,these silicas show no visible discoloration in silicone rubber. Theanalytical data for the oxidatively heat-treated precipitated silica aregiven in Table 1.3.

TABLE 1.3 Analysis: Exp. 1 Exp. 2 Exp. 3 MeOH wettability 63 63 63 Ccontent after heat treatment 3.96 3.47 3.39 Reflectance 92.8 94.5 94.9

The disclosure of German priority application Serial Number 101 38 491.2filed Aug. 4, 2001 is hereby incorporated by reference into the presentapplication.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A process for preparing hydrophobic precipitated silica, whichcomprises: a) preparing a mixture of an organopolysiloxane derivativeand a precipitated silica; b) conditioning the mixture obtained at atemperature ranging from 10 to 150° C. for a period ranging from 0.5 to72 h, and c) conducting oxidative heat treatment under fluid conditionsat a temperature more than 300° C. with an oxidizing gas.
 2. The processas claimed in claim 1, wherein, in a), the organopolysiloxane derivativeis added to a precipitated silica having a water content ranging from1.0 to 80% by weight.
 3. The process as claimed in claim 1, wherein a)occurs by adding the organopolysiloxane derivative to a precipitatedsilica having a water content ranging from 70 to 99% by weight, withsubsequent isolation of the solid from the water.
 4. The process asclaimed in claim 1, wherein a) occurs by simultaneously supplying aprecipitated silica or hydrous precipitated silica and theorganopolysiloxane derivative to a spin-flash drier.
 5. The process asclaimed in claim 1, wherein a) is conducted by mixing a precipitatedsilica with a ready-conditioned precipitated silica prepared accordingto a) and b).
 6. The process as claimed in claim 1, wherein b) occurs byheat treatment at a temperature ranging from 100 to 150° C. for from 0.5to 2 hours.
 7. The process as claimed in claim 1, wherein b) occurs bystorage at room temperature for at least 48 hours.
 8. The process asclaimed in claim 1, wherein the oxidizing gas comprises Cl₂, NO₂, N₂O₅,NO, N₂O, O₃, O₂, Br₂ and/or F₂.
 9. The process as claimed in claim 8,wherein the oxidizing gas further comprises an inert gas.
 10. Theprocess as claimed in claim 9, wherein the amount of oxidizing gas inthe inert gas is at least 1% by volume.
 11. The process as claimed inclaim 1, wherein the oxidizing gas is air or a mixture of air with inertgases.
 12. The process as claimed in claim 1, wherein the oxidizing gasfurther contains up to 80% by volume of water.
 13. The process asclaimed in claim 1, wherein one of a), b), and c) is conducted a numberof times in succession.
 14. The process as claimed in claim 1, whereina) and b) are conducted a number of times in succession.
 15. The processas claimed in claim 1, wherein a), b), and c) are conducted a number oftimes in succession.
 16. The process as claimed in claim 1, wherein thefluid conditions are set in a suspended bed, moving bed, fluidized bedand/or turbulent bed.
 17. The process as claimed in claim 1, wherein theorganopolysiloxane derivitive is selected from the group consisting of(ii) [SiR_(x)X_(y)O]_(z), wherein 0≦x≦2, 0≦y≦2, 3≦z≦10 and x+y=2; (iv)SiR_(n)X_(m)OSiR_(o)X_(p), wherein 0≦n≦3, 0≦m≦3, 0≦o≦3, 0≦p≦3, n+m=3 ando+p=3; and (vi) SiR_(n)X_(m)[SiR_(x)X_(y)O]_(z)SiR_(o)X_(p), wherein0≦n≦3, 0≦m≦3, 0≦x ≦2, 0≦y≦2, 0 ≦o≦3, 0≦p≦3, 1≦z≦10,000, n+m=3, x+y=2 ando+p=3, or the organopolysiloxane derivitive is produced from a precursorselected from the group consisting of (i) R_(4-n)SiX_(n), wherein n=1, 2or 3; (iii) [SiR_(x)X_(y)N]_(z), wherein 0≦x≦2, 0≦y≦2, 3≦z≦10 and x+y=2;an (v) SiR_(n)X_(m)NSiR_(o)X_(p), wherein 0≦n≦3, 0≦o≦3, 0≦m≦3, 0≦p≦3 andn+m=3, o+p=3, wherein R comprises alkyl and/or aryl radicals, andwherein X comprises a reactive group.
 18. The process as claimed inclaim 17, wherein the organopolysiloxane derivative isSiR_(n)X_(m)[SiR_(x)X_(y)O]_(z)SiR_(o)X_(p), wherein 0≦n≦3, 0≦m≦3,0≦x≦2, 0≦y≦2, 0≦o≦3, 0≦p≦3, 1≦z≦10,000, n+m=3, x+y=2 and o+p=3, whereinR is methyl.
 19. The process of claim 17, wherein X is selected from thegroup consisting of silanol, amino, mercapto, halide, alkoxy, alkenyland hydride.
 20. The process of claim 1, wherein the hydrophobicprecipitated silica has the following properties: carbon content >3.1%methanol wettability  >60% reflectance  >94% BET/CTAB ratio >1 and <3DBP absorption <230 g/100 g BET surface area 50–110 m²/g CTAB surfacearea >30 m²/g water vapor absorption at 30° C. and 1.1 ± 0.2% 30 ambienthumidity water vapor absorption at 30° C. and 1.4 ± 0.3% 70 ambienthumidity.


21. A hydrophobic precipitated silica prepared by the process accordingto claim
 1. 22. The hydrophobic precipitated silica according to claim21, which has the following properties: carbon content >3.1% methanolwettability  >60% reflectance  >94% BET/CTAB ratio >1 and <3 DBPabsorption <230 g/100 g BET surface area 50–110 m²/g CTAB surfacearea >30 m²/g water vapor absorption at 30° C. and 1.1 ± 0.2% 30 ambienthumidity water vapor absorption at 30° C. and 1.4 ± 0.3% 70 ambienthumidity.


23. A process for preparing hydrophobic precipitated silica, whichcomprises: a) preparing a mixture of an organopolysiloxane derivativeand a precipitated silica; b) conditioning the mixture obtained at atemperature ranging from 50 to 80° C. for a period ranging from 15 to 30h, and c) conducting oxidative heat treatment under fluid conditions ata temperature more than 300° C. with an oxidizing gas to produce ahydrophobic precipitated silica.
 24. The process of claim 23, whereinthe hydrophobic precipitated silica has the following properties: carboncontent >3.1% methanol wettability  >60% reflectance  >94% BET/CTABratio >1 and <3 DBP absorption <230 g/100 g BET surface area 50–110 m²/gCTAB surface area >30 m²/g water vapor absorption at 30° C. and 1.1 ±0.2% 30 ambient humidity water vapor absorption at 30° C. and 1.4 ± 0.3%70 ambient humidity.


25. A process for preparing hydrophobic precipitated silica, comprising:a) preparing a mixture of an organopolysiloxane derivative and aprecipitated silica; b) conditioning the mixture obtained at atemperature ranging from 10 to 150° C. for a period ranging from 0.5 to72 h, and c) conducting oxidative heat treatment under fluid conditionsat a temperature of more than 300° C. with an oxidizing gas to produce ahydrophobic precipitated silica.
 26. The process of claim 25, whereinthe hydrophobic precipitated silica has the following properties: carboncontent >3.1% methanol wettability  >60% reflectance  >94% BET/CTABratio >1 and <3 DBP absorption <230 g/100 g BET surface area 50–110 m²/gCTAB surface area >30 m²/g water vapor absorption at 30° C. and 1.1 ±0.2% 30 ambient humidity water vapor absorption at 30° C. and 1.4 ± 0.3%70 ambient humidity.